1. Field of the Invention
This invention relates to an electronic balance, and particularly to an electronic balance which is provided with a Roverbal mechanism (also called as a parallel guide), generates an electromagnetic force to eliminate displacement of a displaced portion due to a measuring load, and detects the measuring load from the generated electromagnetic force.
2. Description of the Related Art
In an electronic balance, generally, electromagnetic force is generated against the displacement of a sensor mechanism body due to measuring load, and the measuring load is measured on the basis of the electromagnetic force required for the displacement to be zero. As the sensor mechanism body in such electronic balance, a mechanism body provided with what is called a Roverbal mechanism is employed, in which a movable pillar supporting a measuring pan on which a measuring load is placed is supported on a fixed pillar by two beams parallel to each other each of which provided with flexible portions at both ends (refer to JP-A-2002-148105 and JP-A-2004-28750). The provision of the Roverbal mechanism prevents the measuring pan from tilting due to the measuring load, and restricts the displacement of the measuring pan or the movable pillar due to the measuring load in a vertical direction. Thus, the load detecting error due to the position of the measuring load placed on the measuring pan can be eliminated, thereby permitting the measurement with high accuracy.
FIG. 3A shows a side view of a load measuring cell 20 machined from a unit block which is employed as the above sensor mechanism body. FIG. 3B shows a top view thereof in which a measuring pan 29 is not shown. FIG. 3C shows a partial sectional view of an electromagnetic force generating device 30. As seen from these figures, the load measuring cell 20 forms, from a unit block shape, flexible portions e and beams 23a, 23b which configures a Roverbal mechanism two-dimensionally with wire electric discharge machining, and a lever block 26 which connects to power-point springs 24a, 24b and a supporting-point spring 25. Also, in the load measuring cell 20 as shown in FIG. 3B, widths in x-direction of the power-point springs 24a, 24b and the supporting-point spring 25 are narrowed by forming spot facings 27 on both side portions in a longitudinal direction of the load measuring cell 20. Thus the mechanical sensitivity is improved. As shown in FIG. 3A, the measuring pan 29 is fixedly attached to the upper end of a movable pillar 28 of the load measuring cell 20. A lever 31 with a force coil 30a of the electromagnetic force generating device 30 fixed at its one end is screwed on both sides of the lever block 26. This load measuring cell 20 has features such as that it can reduce the number of components and provide a high reproducibility of temperature characteristic.
The electronic balance in the related art is constructed as described above, but has the following problem. The lever block 26 is likely to vibrate according to the vibration in a width direction (or x-direction as shown in FIG. 3B) of the load measuring cell 20, i.e. in a direction perpendicular to the movable direction of the lever block 26. Accordingly, indicated value is varied for temporary vibration, and an error is generated continuously for steady vibration. Further, it was difficult to arrange the power point and the supporting point in their ideal shape, and implement the mechanical sensitivity in an optimum design.